This invention is directed to a cryogenic system where both the compressor and expander, which operate with the cryogenic refrigerant fluid in the system, are rotary screw-type machinery of the Lysholm type with the expander being coupled to the drive system with a magnetic coupling operating through a wall to separate the compressor and expander chambers.
Lysholm built an early prototype of the rotary screw compressor in 1934. Some of his development work was described in the Proceedings of the Institution of Mechanical Engineers, Vol. 150, No. 1, Pages 11-16 and 4 plates 1943. One of the main features of this screw compressor is the fact that it can run without oil or other lubricant in the compression chamber. No oil is necessary because the rotors do not contact each other or the casing. The only mechanical contact is in the bearings and in the timing gears (if any) which can be located on the outside of the gas containing casing and away from the refrigerant gas flow stream.
The Lysholm type rotary screw compressor has two rotors with intermeshing lobes. Within the intermesh of the lobes and housing, the compression takes place. Two helical rotors comprise the working parts of the screw compressor. The male rotor generally has four lobes and rotates 50 percent faster than the female rotor which has six flutes between which are grooves in which the lobes interengage. Other ratios of lobes to flutes are also used. The gas is compressed in the spaces between the housing, the lobes and the grooves. The lobes and the grooves are helical so that the space appears to move progressively toward the outlet end of the housing, and the space becomes progressively smaller along the length of the rotors as the rotors rotate. Thus, gas taken in the inlet port at the suction end is compressed in the space as the rotors turn and the gas is delivered at higher pressure from the outlet port at the delivery end of the housing. The inlet and outlet ports are automatically covered and uncovered by the shaped ends of the rotors as they turn.
There has been considerable development work done on the improvement of such screw compressors. Most of the patents are owned by Svenska Rotor Maskiner which devoted the pioneer effort in this art and appears to hold most of the patents. The company is located in Nacka, Sweden.
Nilsson, U.S. Pat. No. 3,245,612 and Schibbye, U.S. Pat. Nos. 3,283,996 and 3,423,017 are particularly directed to the shapes of the lands and the grooves in the rotors, but show the porting and general organization of the rotary screw compressor to show how compression and expansion are achieved in such a structure. Furthermore, this type of screw compressor is illustrated as being the compressor in refrigerator systems in U.S. Pat. Nos. 3,432,089; 3,811,291; 3,848,422 and 3,945,216. While the use of screw compressors has been recognized for refrigerator compressor service, the use of such devices as expanders for such service has not been recognized. Furthermore, it has not been previously recognized that screw compressors and expanders in the same refrigerator can efficiently run at about the same speed so that they can be coupled directly or through gearing, for speed control of the expander and for power feedback from the expander. In the refrigeration arts, it is known that with source gases and conditions it is necessary to extract work during expansion to produce refrigeration, with some refrigerant gases within some of their operating temperature ranges. In the past, piston expanders have been used, usually in smaller refrigerators, and turbo expanders have been used, usually in large refrigerators. While the work output of such expanders is not significant in terms of total refrigeration input power, speed control of the expander is necessary. Such speed control has been difficult where the turbo expander runs at very high speed. It is part of this invention that the employment of an expander coupled to and running with the compressor is feasible when screw-type equipment is used for refrigerant gas compression and expansion.
In order to supply economical refrigeration a design of minimum complexity, associated with long and trouble free life, is necessary. The structure of this invention provides a refrigerator which is of low weight per unit of refrigeration, and is especially designed so that in the small sizes for which this refrigeration system is most suitable, the structure is of simplified mechanical design. Thus, such a refrigeration system can be used to cool devices for long maintenance free life and can be employed in locations where total weight and input power should be minimized.